SMTP is a protocol, or language. It isn't an application running on your server. Here's email in a nutshell...
- The sender uses a "Mail User Agent" (MUA) to compose a message. The MUA then connects to a "Mail Transport Agent" (MTA) via the "Simple Mail Transport Protocol" (SMTP) in order to relay the message to its recipient.
- The MTA queries the DNS for the MX records corresponding to the domain part of the recipients email address.
- The MTA originates an outbound connection via SMTP to the target MTA listed in the MX record. If the target MTA accepts the message, the originating MTA is done and the target now has responsibility for the message. If not, then the originating MTA requeues the message for re-transmission.
- The target MTA hands of the message to a "Mail Delivery Agent" (MDA).
- The MDA writes the message to the mail spool in either mbox or maildir format. mbox is the default in RHEL. The MDA could use a database in advanced installations.
- On the receiving end, the recipient uses a MUA to query a "Mail Retrieval Agent" (MRA) for new mail. The two prevailing protocols for checking mail are POP and IMAP. Dovecot and CYRUS are MRA packages.
- The MRA accesses the user's mail spool and returns the messages.
So, what does all that mean?
- Thunderbird and Evolution are a MUAs
- Sendmail and Postfix are MTAs
- Cyrus and Dovecot are MRAs
- Procmail and maildrop are MDAs
Has that helped?